A memory device such as an SSD (solid state drive) which has non-volatile semiconductor memory typically starts a process of writing data which are requested to be written by a write command received from an external device, without overwriting data already written into the non-volatile semiconductor memory. Such a memory device may need to prepare, in advance, free physical blocks into which data have not been written, for the data writing. However, when the free physical blocks remain unwritten for a long period of time, reliability of data that are written into the free physical blocks may decrease.
Moreover, garbage collection may be carried out with respect to a physical block that still has an unwritten region therein. In such a case, an erase process is carried out to the unwritten pages of the physical block. However, when the erase process is carried out on the unwritten pages, reliability of data that are written into the unwritten pages in a later write process may decrease.